1. Field of the Invention
The present invention relates to a method of making a tunnel junction sensor with a smooth interface between a pinned or free layer and a barrier layer and, more particularly, to such a method which reduces a coupling field between the pinned and free layers.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm urges the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a tunnel junction sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes an insulative tunneling or barrier layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90° to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The tunnel junction sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers, are connected to the tunnel junction sensor for conducting a tunneling current therethrough. The tunneling current is conducted perpendicular to the major film planes (CPP) of the sensor as contrasted to a spin valve sensor where the sense current is conducted parallel to or parallel to the major film planes (CIP) of the spin valve sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, occurs when the tunneling current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the tunnel junction sensor to the tunneling current (IT) is at a minimum and when the magnetic moments are antiparallel the resistance of the tunnel junction sensor to the tunneling current is at a maximum. Changes in resistance of the tunnel junction sensor is a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layers. When the tunneling current (IT) is conducted through the tunnel junction sensor resistance changes, due to field signals from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the tunnel junction sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel junction sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the tunnel junction sensor at minimum resistance. The dr/R of a tunnel junction sensor can be on the order of 40% as compared to 10% for a spin valve sensor.
Tunnel junction sensors are classified as either a top tunnel junction sensor or a bottom tunnel junction sensor. In a bottom tunnel junction sensor the pinning layer is closer to the first shield layer than the second shield layer and in a top tunnel junction sensor the pinning layer is closer to the second shield layer than to the first shield layer. In either type of sensor the first and second shield layers may engage the bottom and the top respectively of the tunnel junction sensor so that the first and second shield layers serve as leads for conducting the tunneling current through the tunnel junction sensor perpendicular to the major planes of the layers of the tunnel junction sensor. The tunnel junction sensor has first and second side surfaces which are normal to the ABS. First and second hard bias layers abut the first and second side surfaces respectively for longitudinally biasing the magnetic domains of the free layer. This longitudinal biasing maintains the magnetic moment of the free layer parallel to the ABS when the read head is in the quiescent condition.
An inherent characteristic of the tunnel junction sensor is the existence of a ferromagnetic or antiferromagnetic coupling field between the pinned and free layers across the barrier layer. Unfortunately, this coupling field urges the magnetic moment of the free layer from its parallel position to the ABS in the quiescent condition. Accordingly, it is desirable that the coupling field be minimized in the tunnel junction sensor. In the spin valve sensor this is accomplished by providing one or more thick oxide seed layers which improve the microstructure of the layers deposited thereon which, in turn, can be employed for minimizing the coupling field. Unfortunately, this approach cannot be employed in reducing the coupling field in a tunnel junction sensor since the tunneling current is conducted perpendicular to the major thin film planes of the sensor and the thick oxide seed layers would increase the resistance of the tunnel junction sensor to the tunneling current thereby reducing sensitivity of the sensor to field signals. Another approach to reduce the coupling field in a tunnel junction sensor is to increase the thickness of the oxide barrier layer. This increases the distance between the pinned and free layers so that the coupling field is reduced. Unfortunately, this approach is not practical since an increase in the thickness of the oxide barrier layer again increases the resistance of the sensor to the tunneling current which, in turn, reduces the sensitivity of the sensor. Accordingly, there is a strong-felt need to provide a tunnel junction sensor with a low resistance and a low coupling field.